1. Field of the Invention
The invention relates generally to a process for improving coal to make it less susceptible to spontaneous combustion. Particularly, the process of the invention relates to coating coal particles with an incombustible coating, specifically, silicon dioxide.
2. Status of the Industry
The rapid growth of electrical demand in the United States, the concern for low sulfur dioxide emissions from power plants, and the national goal for energy self-sufficiency have all fostered an unprecedented boom in Western coal. This low-rank, low sulfur, high-moisture coal is now shipped by unit trains carrying 10,000 tons of coal thousands of miles to Eastern and Southern electrical utilities.
There are certain disadvantages in shipping and storing low rank coal. First of all, Western coals frequently contain 25% or more inherent moisture, and the shipping cost for transporting this much moisture in large shipments is significant. Secondly, all coals and especially low-rank coals have a tendency to ignite spontaneously during shipping or storage. Thirdly, low-rank coals are usually quite dusty, even through they may contain 25% inherent moisture.
3. Prior Art
In an article entitled "Self-Heating of Carbonaceous Materials" by F. L. Shea, Jr., and H. L. Hsu, Great Lakes Research Corporation, Elizabethton, Tennessee, the authors reported the development of a simple test for the determination of the self-heating rates of various carbonaceous materials. They state that their test, which provides for contact between oxygen at 150.degree.F (66.degree.C) saturated with water vapor and materials which contain sufficient hydrocarbons to combine with oxygen at around 300.degree.F (149.degree.C) and thus ignite at relatively low temperatures, provides a good measure of the liability of the material to spontaneously ignite. The effect of moisture in the self-heating process is extremely important. This was well illustrated by the behavior of raw lignite which, in the presencee of dry oxygen at 150.degree.F (66.degree.C) experienced a temperature rise of only 18.degree.F (10.degree.C) inabout five hours while in the presence of oxygen saturated with water vapor at 150.degree.F (66.degree.C) the rate of temperature rise was on the order of tenfold greater, with the lignite igniting in less than 5 hours.
U.S. Pat. No. 3,723,079 discloses a process for "stabilizing" dried lignitic and subbituminous coals against spontaneous combustion, in which the dried coal is treated at about 347.degree.F (175.degree.C) to 437.degree.F (225.degree.C) with oxygen followed by some rehydration of the oxygen-treated coal with water. The test, which the inventors used to measure the stability of the coals treated by their process described in the above patent, was essentially the same test as developed by Shea and Hsu as described above.
While coal can be thermally dried below its inherent moisture value, this has disadvantages. The product has perhaps a even greater tendency toward spontaneous ignition than undried coal, it is dustier, and under humid conditions it will reabsorb moisture.
U.S. Pat. No. 3,723,079 described previously, discloses a process for stabilizing a coal against spontaneeous combustion; however, the treatment requires addition of water which materially offsets the advantages of drying.
U.S. Pat. No. 3,014,815 discloses a process for coating articles with a metal oxide by introducing into a coating chamber containing the articles a hydrolysable metal compound,, which may be the chloride of the metal, oxygen, and hydrogen or a compound which produces hydrogen. The chloride is hydrolysed by the water formed to produce the metal oxide with which the article is coated. The patent does not disclose coating coal and does not disclose the use of silicon dioxide as coating material. Further, and most important, the source of water required for the process is oxygen and a compound which reacts with oxygen to form water; the formation of water by this method requires heating the articles to at least approximately 1112.degree.F (600.degree.C).